The synthetic wood products produced by extruding molding materials obtained by melt-kneading thermoplastic resin, wood meal, and other secondary materials added as required into a desired shape have characteristics of resin moldings, such as resistance to decomposition while having their wood textures. Accordingly, the synthetic wood products are widely used, for example, as building materials for wood decks that are installed outdoors after being processed into boards or the like.
In the manufacturing of such synthetic wood products, thermoplastic resin, wood meal, and other secondary materials cannot be extruded satisfactorily if delivered directly into the cylinder of the extruder provided in an extrusion apparatus for manufacturing synthetic wood products because pyroligneous acid or moisture contained in the wood meal produces a large amount of gas in the cylinder of the extruder.
Furthermore, even without such occurrence of gas, a large extruder will be necessary if an attempt is made to melt-knead thermoplastic resin, wood meal, and other secondary materials until they are uniformly distributed.
For this reason, raw materials are usually not delivered directly into an extruder when synthetic wood products are to be manufactured. Instead, raw materials are kneaded and compounded in advance, then the compounded raw materials are granularized into pellets (pellets produced by compounding two or more different types of raw materials are referred to as “composite pellets” throughout the present invention), and finally the composite pellets obtained in this manner are used as molding materials to be extruded into synthetic wood products.
As one example of a method for manufacturing such composite pellets, there is proposed a method of manufacturing in a batch composite pellets used to extrude into synthetic wood products, where wood meal is dried and pyroligneous acid gases are volatilized using heat generated at the time of stirring with a Henschel mixer; kneaded materials are produced by melt-kneading the raw materials; granularized materials with a certain grain size are produced by stirring these kneaded materials while being cooled with a cooling mixer; and then the granularized materials are further crushed into smaller pieces with a cutter mill (refer to Patent Document 1).
Because batch manufacturing methods, as described above, have low productivity, there is also proposed another example of a method of manufacturing chip-shaped or pellet-shaped extrusion molding materials by introducing into a die kneaded materials extruded by an extruder to mold the materials into a sheet or strand (circular belt) shape and then cutting the extruded sheet-shaped or strand-shaped kneaded materials.
In this kind of manufacturing of extrusion molding materials with an extruder, because a large amount of gas is produced in the cylinder of the extruder due to pyroligneous acid or moisture contained in wood meal, pre-kneading with the extruder is proposed for example, by providing a vent hole in the cylinder of the extruder and sucking the gases produced in the cylinder via this vent hole (Patent Documents 2 to 5).